Nonaqueous electrolyte secondary batteries represented by lithium secondary batteries with a high energy density and high capacity are widely used as a power supply to drive modern portable electronic devices such as cellular phones, portable personal computers, and portable music players and further used as a power supply for hybrid electric vehicles (HEV) and electric vehicles (EV). In the nonaqueous electrolyte secondary batteries used particularly for small devices, a prismatic type is widely used because of its advantages in space efficiency and easy handling.
These nonaqueous electrolyte secondary batteries generally include a positive electrode plate in which a positive electrode substrate is made of aluminum foil or the like in elongated sheet, and both surfaces of the positive electrode substrate are coated with a positive electrode mixture that includes a positive electrode active material absorbing and desorbing lithium ions; and include a negative electrode plate in which a negative electrode substrate is made of copper foil or the like in elongated sheet, and both surfaces of the negative electrode substrate are coated with a negative electrode mixture that includes a negative electrode active material absorbing and desorbing lithium ions. The positive electrode plate and the negative electrode plate, which are made insulated from each other by interposing therebetween a separator made of microporous polyethylene film or the like, are wound in cylindrical or in oval to form a wound electrode assembly. In the case of a prismatic battery, the wound electrode assembly is thereafter pressed into a flattened shape.
Patent Document 1 discloses a prismatic nonaqueous electrolyte secondary battery including such a flattened wound electrode assembly. As an example of the prismatic nonaqueous electrolyte secondary battery in related art, the structure of a prismatic sealed battery 50 disclosed in Patent Document 1 will now be described with reference to FIG. 7.
FIG. 7 is a perspective view of a main portion of the prismatic sealed battery 50 disclosed in Patent Document 1. The prismatic sealed battery 50 has a structure that a battery outer housing 52, made of aluminum, concurrently serving as a positive electrode external terminal accommodates therein a spiral electrode assembly 51 having a positive electrode plate and a negative electrode plate, which are wound together with a separator interposed therebetween.
The positive electrode plate has a positive electrode substrate exposed portion that has no positive electrode active material layers on either surface of the positive electrode substrate (hereinafter “both surfaces exposed portion”) for a certain distance from the winding terminating end of the positive electrode substrate. Furthermore, another positive electrode substrate exposed portion, in which either surface of the positive electrode substrate has a positive electrode active material layer and the other surface has the substrate exposed (hereinafter the “either surface exposed portion”), is provided for a certain distance from the both surfaces exposed portion. The both surfaces exposed portion has a substantially U-shaped slit portion formed with three slit lines penetrating the substrate, and a positive electrode collector tab 54 is formed by cutting and raising up the slit portion.
The spiral electrode 51 is wound in a manner that the either surface exposed portion of the positive electrode plate faces to the outer periphery and the both surfaces exposed portion is arranged at the outermost periphery of the spiral electrode 51. Doing so makes the substrate exposed portion and the inner surface of the battery outer housing in contact with each other. By the contact of the substrate exposed portion and the inner surface of the battery outer housing, the battery outer housing 52 that concurrently serves as a positive electrode external terminal and the positive electrode are made electrically connected to each other.
The battery outer housing 52 is sealed by a sealing plate 53 welded along the edge of the upper mouth by laser welding. The outer periphery side surface of the sealing plate 53 and the inner surface of the mouth marginal portion of the battery outer housing 52 are welded by laser, holding therebetween the positive electrode collector tab 54 derived from the positive electrode plate.
In the negative electrode plate, a negative electrode collector tab 55 is electrically connected by spot welding to a negative electrode substrate exposed portion, which is formed at the winding starting end. Furthermore, the negative electrode collector tab 55 is electrically connected to a collector terminal plate 56 fixed to the sealing plate 53. An adhesive tape 57 is attached to the winding terminating end portion of the spiral electrode assembly 51 so as to protect the positive electrode collector tab 54 that is cut and raised up.
As above explained. with the sealed battery 50 disclosed in Patent Document 1, the positive electrode collector tab 54 is formed of a piece of foil that is made by cutting and raising up a part of the positive electrode substrate exposed portion. Thus, the properties of the positive electrode collector tab 54, such as mechanical strength, depend on the properties of the material used for the positive electrode substrate. However, because aluminum foil in the order of 10 and some micrometers is usually used for a positive electrode substrate, the sealed battery 50 disclosed in Patent Document 1 has a problem in that the positive electrode collector tab 54 has reduced mechanical strength.
As a solution of the above problem to improve the mechanical strength of the positive electrode collector tab, Patent Document 2 discloses an invention of a prismatic sealed battery that has a positive electrode tab made by electrically connecting a piece of foil with higher mechanical strength than the positive electrode substrate to the cut-raised up piece, instead of using the cut-raised up piece made by cutting and raising up the positive electrode substrate directly as a positive electrode collector tab.